Dispensing devices are known which produce a finely divided spray of liquid droplets by electrostatic (more properly referred to as `electrohydrodynamic`) means. The droplet spray in such devices is generated by the application of an electric field to a liquid at a spray head or spray edge. The potential of the applied electric field is sufficiently high to provide comminution of the liquid from the spray head. The droplets produced are electrically charged and thus are prevented from coagulating by mutual repulsion.
Electrohydrodynamic sprayers have potential uses in many areas, including agriculture and the automotive industry and also for dispensing cosmetics and medicines.
United Kingdom patent number 1569707 describes such an electrohydrodynamic spray device principally for use in crop spraying.
United Kingdom patent number 2018627B discloses an electrohydrodynamic spray device wherein the charged droplet spray is fully or partially electrically discharged by means of an earthed electrode having a sharp or pointed edge and located downstream of the spray head. European Patent number 0234842 also uses this technology and relates to an inhaler in which charged droplet spray is discharged prior to inhalation by means of a sharp or pointed discharge electrode carrying an opposite charge to the droplet spray and located downstream of the spray head. The droplets are discharged to facilitate droplet deposition into the respiratory tract by preventing deposition of charged droplets onto the mouth and throat of the user.
A common feature of all known electrohydrodynamic spray devices is that the electric charge used to generate the spray is applied directly to the spray head. It has now surprisingly been found that the direct application of the field is not essential and that the electrohydrodynamic comminution of a liquid may be accomplished by inducing the required electric charge at the spray head. In addition and advantageously, it has been found that the comminutions produced can be partially or fully discharged prior to use.
This method of induced charging has been found to provide better comminution of liquids having lower electrical resistivity.